The present invention relates to a multiplex demodulation circuit for use in an FM stereophonic tuner.
A multiplex demodulation circuit operates to separate right and left channel signals from a so-called "composite signal" produced by an FM detector. In such a multiplex demodulation circuit, in general, a 19 KHz pilot signal included in the composite signal is detected to obtain a 38 KHz square wave subcarrier signal and the composite signal is switched by the square wave subcarrier signal to separate and provide the right and left channel signals. Because it is in the form of a square wave as described above, this switching signal includes odd harmonics of 38 KHz. Accordingly, if a signal whose frequency is close to the frequencies of the odd harmonics is mixed in the FM detection output signal, beating occurs between the mixed signal and the odd harmonics of 38 KHz in the switching signal as a result of which unwanted audio components are included in the demodulation output.
In order to overcome this difficulty, a band elimination filter for eliminating the above-described mixed signal is inserted between the FM detector and the multiplex demodulation circuit. However, the use of such a filter adversely affects the level and phase characteristic of the demodulated audio sub signal (L-R) and especially the separation in the high frequency range.
Accordingly, an object of the invention is to provide a multiplex demodulation circuit which provides excellent output characteristics without the use of a band elimination filter which adversely affects the signal response.